The invention relates to a method and an apparatus for limiting the top rpm in an internal combustion engine. It more especially relates to engines in which the fuel quantity required per cycle is provided in dependence on rpm and aspirated air quantity by a fuel injection system or a mixture preparing device such as a carburetor.
The majority of internal combustion engines is so constructed that when they are operated at full throttle in the lower gears, or if full throttle is maintained in downhill operation, the engine may reach rpm domains which exceed the design limits and engine damage such as breakage of valves, overheating and the like may result from this operation. Accordingly, the top rpm of the engine should be limited.
A known method for limiting the rpm of an internal combustion engine provides short-circuiting the ignition by means of a centrifugal switch which is usually part of the distributor mechanism.
This process is relatively coarse because of the unavoidable mechanical tolerances and also because there must be a mechanical movement prior to action so that, in general, this known device can interrupt ignition only in a given range of the engine rpm.
Furthermore, the interruption of ignition prevents the combustion of the mixture in the cylinders, but does not prevent the production of a flammable mixture which passes into the exhaust system and collects there. At the moment when ignition reoccurs, this flammable mixture in the exhaust system may combust with the obvious detrimental results. If the engine is equipped with an after-burner or catalyzer, the known method for limiting the rpm by interrupting the ignition is particularly detrimental because the uncombusted fuel-air mixture will heat the catalyzer and may lead to its destruction.